I wanted to see the unit file because it has changed several times during the lifetime of CentOS 7.0 - 7.6 and I wondered if you had an older copy. You don't, it's the same as the sample one included in the package.

So, I am not sure why it would be doing this for you but I do have one suggestion about how to bypass it but it depends on how much you use /tmp. There is another systemd unit file that is disabled by default - tmp.mount. If that's enabled using systemctl enable tmp.mount then systemd will mount /tmp on tmpfs and allocate it by default to be half the size of your RAM. Don't worry, it won't necessarily use all that as it only uses the amount of RAM that's the size of the files you place on it. It's also swap backed so the data can be swapped out if the RAM is required for something more useful.

Benefits of using tmpfs for /tmp - it's faster, it starts empty on each reboot. Reasons not to use it include needing to preserve the contents of /tmp over reboots and requiring more space than you have RAM+swap.

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